1. Field of the Invention
This invention relates to threshold voltage modulation in FET semiconductor devices.
2. Brief Description of the Prior Art
The threshold voltage, VT, of a field effect transistor (FET) depends upon the voltage on the substrate or body region (the region between the source region and the drain region). The threshold voltage can be changed by changing the body voltage. For an n-channel transistor, an increase in body voltage lowers (makes less positive or more negative) the threshold voltage. For a p-channel transistor, an increase in the body voltage raises (makes more negative or less positive) the threshold voltage, VT. Accordingly, for an n-channel transistor, when the gate voltage goes high, the capacitive coupling to the body region raises the body voltage and lowers the threshold voltage. This provides increased drive current. When the gate is turned off, the gate coupling raises the threshold voltage by lowering the body voltage, thereby providing decreased leakage current when the transistor is off. This capacitive coupling of the gate to the body is significant only when the conductive channel (inversion region) from the source region to the drain region is not formed. When the conductive channel is formed, it electrically isolates the gate from the body. Therefore, after the channel is formed, the advantage of the capacitive coupling of the gate to the body is no longer improved.
In the prior art, the edges of a transistor were sometimes doped more heavily to prevent leakage at the edge. The more heavily doped edge region would have a higher VT and thus the gate-to-body capacitive coupling would exist at the edge region for a greater range of gate voltage than at the channel region. However, the more heavily doped region would be made as small as practical, and a sidewall dielectric thicker than the gate. oxide would generally be used. A sidewall insulator is required with mesa isolation. With shallow trench isolation (STI), the space between sidewalls is filled with dielectric. In the prior art, the sidewall insulator is typically several time the thickness of the gate oxide. Thus, the contribution of the highly doped edge region to the gate-to-body coupling would not be substantial.
Assuming, for example, an n-channel transistor, coupling of the gate to the body of the transistor causes dynamic VT modulation, thereby increasing VT when the transistor is off and decreasing VT when the transistor is on. However, direct coupling from the gate to a body contact results in gate current and diminished chip area. These problems have been attacked in the prior art by providing separate capacitors which are connected between the gate and the body of the transistor. This eliminates gate current but still diminishes chip area. Also, resistance in the connection of the capacitor to the body diminishes the effectiveness of the coupling. It is therefore desirable to increase the capacitive coupling between the gate and the body of a FET as well as to do so in a more economical, more effective and simpler procedure.
The above noted desires are readily accomplished in accordance with the present invention in a simple and economical manner.
Briefly, to provide an enhancement of on current and a suppression of leakage current with voltage change on the gate of an FET, the capacitive coupling between the gate and the body of the FET is increased by providing a region under the gate, contiguous with the channel region, having a much higher threshold voltage than the channel region. This region may be adjacent to one or both edges of the channel region, going from source to drain. For silicon on insulator (SOI) transistors with a mesa structure, the high VT region may be adjacent to a mesa edge (referred to as a transistor sidewall). Here, higher VT means more positive (less negative) for n-channel and more negative (less positive) for p-channel. Alternatively, the high VT region may be anywhere along the width of the transistor and may be encompassed within the low VT region. Multiple high VT regions may be distributed within the channel region.
Coupling of the gate to the body is further increased by using a leaky gate dielectric. However, gate leakage through the gate oxide can be to the body region only when the channel is not formed. Thus, this method of influencing the body voltage is more effective in a high VT region. Having a thin gate dielectric also has well known benefits in drive current and reduced short channel effects. However, gate leakage to the source, drain or channel is generally not desirable. A structure with selectively thin (less than 20 nanometers) or leaky gate dielectric over the high VT region maximizes the beneficial influence of the gate coupling to the body while reducing total gate leakage. In the case of the leaky dielectric, the dielectric over the high VT region is leakier than the dielectric over the low VT region. This can be accomplished by, for example, growing a thin oxide, e.g. 15 nanometers,, depositing a nitride and patterning and etching the nitride/oxide stack to expose the low VT regions. The gate oxide is then grown, the remaining nitride is stripped and the procedure progresses in standard manner. Although this arrangement is not the same as a direct contact to the body, the gate current influence on the body potential is beneficial.
The capacitive coupling of the gate to the body provides a floating body with some of the benefits of dynamic VT modulation. This capacitive coupling is reduced by the shielding of the gate from the body by the channel when the channel is formed. It has been suggested in the prior art to build a separate capacitor to capacitively couple the gate to the body. This extra capacitance can be built efficiently in accordance with the present invention by having extra doping in portions of the channel to raise the VT in those regions. Since the high VT region is adjacent to the channel region, there is minimal series resistance between the capacitive coupling to the gate and the body region at the channel. If this capacitance is provided at the edges of the transistor, this will have the extra benefit of suppressing any edge leakage. This can be accomplished in a self-aligned manner using what has been previously described for edge implant, but with a larger offset to provide area for the gate-to-body capacitance.
For example, a nitride is deposited over the pad oxide and patterned and etched for a moat (undersized). Then polysilicon sidewalls are added and the isolation, such as trench isolation or LOCOS, is formed. The polysilicon sidewalls are removed and a channel stop is implanted to form the capacitor area. Standard processing of the transistor, including VT implants, follows. The described process forming and later removing sidewalls on the patterned moat masking layer has the advantage of selfalignment of the high VT regions to the transistor edge. High VT regions interior to the transistor edges can also be formed with the described process sequence by including relatively narrow spaces (less than twice the sidewall width) in the nitride pattern where the high VT regions are to be formed. Alternatively, the moat masking layer can be further patterned after formation of the isolation and prior to the high VT implant. Typically, the species implanted into the high VT region and the low VT region will be the same conductivity type. Other variations of process sequence that can be used to form selective high VT regions, such as the use of counter doping schemes, will become apparent to those skilled in the art. For wide transistors, it is desirable to have a plurality of high VT regions distributed across the width of the transistor. It is not necessary that a high VT region extend to either source/drain region.
The extra gate-to-body coupling is particularly attractive for DRAM word lines (W/L) to couple the body voltage low when the W/L shuts off. It would also couple high when the W/L is on, boosting initial charge transfer, but this will be reduced by recombination, so the net effect will be a lower body voltage when W/L goes off. This will lengthen the time for generation current to cause a forward bias, thus the retention time will be lengthened. A similar benefit applies to other dynamic circuits.
Increased W/L to body capacitance significantly lowers the body node voltage when the W/L turns off, increasing retention times. One way to do this is to use sidewall capacitance. This is accomplished, for example, by providing a substrate having a buried oxide thereover with a silicon mesa formed on the outer surface of the buried oxide, the transistor being formed in the mesa. A gate, such as a word line, is formed wrapping around the sides and outer surface of the mesa, separated from the mesa by a thin dielectric. The transistor channel is formed at the outer surface of the mesa and the higher VT gate-to-body capacitance coupling is formed at the sides of the mesa. The sidewall dielectric is of approximately the same thickness as or, for the leaky dielectric case, thinner than the gate dielectric to provide high capacitance. This form is particularly appropriate for relatively narrow transistors (width less than five times the length) as commonly used as pass gate or access transistors in a memory such as a DRAM. Prior art includes a similar structure, but with thicker sidewall dielectric that reduces the capacitive coupling of the gate to body, or with sidewall VT low enough that sidewall channels form, thereby screening the gate coupling to the body.
The channel region can be formed by masking the body to expose the region of the body to be a high VT Portion of the channel, implanting a dopant of the opposite conductivity type into the high VT portion, exposing the entire channel region and then implanting a dopant of the opposite conductivity type into the entire channel to form the high VT portion of the channel and the low VT portion of the channel in the remaining portion of the channel.
One of the usual problems with the previously described structure with thin sidewall dielectric is the unwanted formation of gate filaments along the bottom corner of the mesa. One way to avoid this is to use a damascene gate process flow. An oxide/nitride stack is formed on the other surface of the silicon film of an SOI wafer. The moat or active region is patterned, the trench is etched to the buried oxide and the upper corners of the mesa are rounded. An optional sidewall implant is performed. A thin oxide is grown on the sidewalls followed by an optional sidewall implant. The top nitride is removed, leaving the oxide. A dielectric is deposited. A trench is patterned and etched for a gate. Sidewalls are optionally added inside the trench to make the gate length (L) shorter and the channel implants are provided. The oxide is stripped and the gate oxide is grown followed by deposition of the gate material. This is followed by a chemical mechanical polish (CMP). The deposited dielectric is then removed and the process proceeds in standard manner. Other process sequences, such as, for example, disposable gate, will become apparent to those skilled in the art.
An integrated circuit may be formed with all the transistors having enhanced gateto-body capacitance. Alternatively, only one type of transistor, e.g., n-channel, may have enhanced gate-to-body capacitance. As a further alternative, enhanced gate-to-body capacitance may be applied selectively to individual transistors of either type.
The clocked transistors of dynamic logic are good candidates for gate to body capacitance. Pull-up transistors (transistors which raise the voltage toward VDD) are also good candidates to have extra capacitance from gate to body. Note that this does not provide a great deal of capacitance on the clock because the gate to body capacitance is in series with the body to substrate capacitance. The capacitance should bring the body voltage lower (higher for p-channel) when off, so this allows a lower VT for the same leakage, as long as the clock is running. If left off on the order of a millisecond, the body will drift up (down for p-channel), letting leakage back up so other techniques for reducing leakage current during standby may still be required.
The invention is applied to both bulk and SOI transistors. For SOI, one configuration in the prior art is to have no direct (ohmic) contact of the body region to a supply or signal voltage. In this configuration, the body voltage responds readily to capacitive coupling. Generally, for bulk and some SOI configuration, the body region is connected directly to a supply or signal voltage. For bulk technology, this is generally referred to as the well voltage. Even in this latter configuration, the transient voltage of a local region of the body can respond to capacitive coupling. The close proximity of the capacitive coupling region to the channel region provided by this invention enhances the effectiveness of the capacitive coupling in modulating the effective VT when there is also an ohmic connection of the body to a signal or supply voltage. This is also true for the embodiment of this invention in which a leaky dielectric is used to separate the gate from the body at a high VT region.
The above description pertains to n-channel devices, it being understood that polarities would be reversed for p-channel devices.